In recent years, global scale environmental destruction such as global warming, ozone layer depletion, acid rain, etc. has become an international problem. There is an urgent need for global environmental pollution countermeasures, and at present, various emissions regulations are enforced in each country. Among them, the problem of discharge of volatile organic compounds (VOC) such as organic solvents into the atmosphere is critical, and therefore, VOC reduction is being advanced under reinforcement of VOC emission regulations.
Heretofore, a coating film has been formed by using a coating material containing an organic solvent, but in promotion for VOC reduction, a powder coating material has now become to be widely used. The powder coating material contains no organic solvent and thus requires no exhaust treatment or waste water treatment at the time of coating, and further can be recovered and reused, whereby the environmental load is extremely low.
In recent years, in various fields such as architecture, automobile, etc., there is a growing demand for a coating film with a hue of metallic tone. Usually for formation of a coating film with a hue of metallic tone, a coating material containing a metallic pigment such as aluminum powder, etc. (hereinafter also referred to as a metallic coating material) is used.
However, in a case where the metallic coating material is a powder coating material, as compared with the case of solvent-type or water-type, the metallic pigment tends to be exposed (bleed out) on the surface of the coating film. Exposure of the metallic pigment deteriorates the appearance of the coating film.
With respect to such problems, for example, a powder coating material composition having the following (1) has been proposed.
(1) A powder coating material composition having 100 parts by weight of a coating material resin powder dry-mixed to from 0.1 to 30 parts by weight of a colored metallic pigment for a powder coating material, in which the surface of the colored metallic pigment having a colored pigment attached on the surface of a base metallic pigment, is covered with a particular polymer and the residual solvent amount is at most 5 mass % (Patent Document 1).
On the other hand, as the powder coating material, an acrylic resin-type powder coating material, a polyester resin-type powder coating material or an epoxy resin-type powder coating material has been mainly used. However, a coating film formed by such a powder coating material is inferior in weather resistance.
With respect to such a problem, for example, a powder coating material composition having the following (2) has been proposed.
(2) A layer separation type powder coating material composition which is a powder coating material obtainable by melt-kneading a mixture comprising from 10 to 50 parts by mass of a fluororesin, from 10 to 90 parts by mass of a polyester resin and from 10 to 50 parts by mass of a pigment, followed by cooling and grinding, and of such a type that when the powder coating material is melted and cured, the fluororesin and the polyester resin will be layer-separated without being compatibilized (Patent Document 2).
In a case where such a layer separation type powder coating material composition is used, it is possible to form, in one-coating operation, a coating film of a two-layer structure comprising a polyester resin layer on the substrate side and a fluororesin layer on the air side, and as the coating film has a fluororesin layer on the air side, it is said to be superior in weather resistance.